courtship behavior of large falcons in captivity

RaptorResearch11(1/2):1-46
COURTSHIP BEHAVIOR OF LARGE FALCONS IN CAPTIVITY
by
Peter H. Wrege
Tom
J. Cade
Sectionof Ecologyand Systematics
Langmuir Laboratory
Cornell University
Ithaca, New York 14853
Abstract
The courtshipbehaviorof captivePeregrineFalcons(Falcoperegrinus),
Gyrfalcons(Falco
rusticolus),PrairieFalcons(Falcomexicanus),and LannerFalcons(Falcobiarmicus)is very
similarin basicform and function.No subspecific
differencesin courtshipbehaviorwere
apparentin Peregrines.
Variationsin the vocalizationsof a speciesare commonand function
to communicate
the intensityof motivation.The seasonal
ontogenyof Peregrine
reproductive behavioris similarin all experiencedpairs.Gradualand consistentshiftsin yearly
development
are evidentduringthe firsttwo yearsof breeding,but usuallya patternappears
to stabilizeby the third year.
Major interspecificdifferences
werefound in the frequencyof aggressive
andnonaggressiveposturesin both ritualizedand nonritualizedDisplaysand in their relativeuseby the
male and female. Evidencefrom the behaviorof captivessupportsthe ideathat femalesare
dominantin the pairrelationship.
The influenceof sizedimorphism
on the development
and
maintenance
of femaledominanceis reflectedboth interspecifically
and intraspecifically
in
the relativefrequencies
of agonisticbehavior.We suggestthat potentiallysevereinjury
resultingfrom aggressive
fighting,combinedwith a pair relationshipdependenton female
dominance,has resultedin a repertoireof postureshighly efficient in communicating
fine
changes
in motivationand a vocalrepertoirethat variescontinuallywith the intensityof
motivation.
Introduction
We havebeenstudyingthe courtshipbehaviorof pairedfalconsin captivityasonepart of
an attempt to understandhow behavioraland physiologicalmechanismsfunction in the
reproductionof raptorialbirds,and how thesemechanisms
areinfluencedby environmental
factors.Pairingand reproductionmay involvespecialproblemsfor highlypredatorybirdsespeciallyin confinement-because
they are usuallysolitaryand pugnacious
much of the
yearand because
they possess
formidablebeaksandfeet, aswellasa strongmotivation,for
killing other animals.These include, in the caseof large falcons,birds similarin sizeand
shapeto themselves.Potentially, a falcon representsa hazard to its mate. Given these
conditions,importantsocialprocesses
mustbe broughtinto play to counteractthesestrong
aggressive
tendencies(Willoughbyand Cade 1964).
Thus, socialadjustmentsthat take place for effectivepair-bondingand integrationbetweenmaleandfemalefalconsareparticularlyrewardingsubjectsfor testingcurrenttheories
aboutpairingandsexualselection
(Brown1975),aswellasfor providing
newhypotheses
for
further study. Such studiescan also contribute much to the basic knowledgeneededto
propagatePeregrineFalconsand other threatenedor endangered
speciesin captivityon a
practicalscale.
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RAPTOR RESEARCH
Vol. 11, No. 1/2
The purposeof this paperis to describethoroughlythe courtshipDisplaysandassociated
behaviorof mated falcons,to assigntentative functionsto them, and to proposesome
hypotheses
aboutthe role of sexualsizedimorphismin the pairingof largefalcons.
Materials and Methods
We have examinedfour speciesof the genusFalco for this study: the Peregrine(Falco
peregrinus),Gyrfalcon (Falco rusticolus),Prairie Falcon (Falco mexicanus),and Lanner
Falcon(Falco biarmicus).The formerthreespecies
breedin North America.LannerFalcons
breed mainly in arid parts of Africa where their ecologycloselyparallelsthat of Prairie
Falcons.Our emphasis
is on comparisons
betweenthe behaviorof Peregrines
and Gyrfalcons.
Observations
were maderegularlyfor four seasons
on captivepairsof Peregrines
and for two
seasons
on captiveGyrfalcons.
Three pairs of Ireregrineswere studied in detail, two from northern Alaska (F. p.
tundrius)and one from the QueenCharlotteIslands(F. p. pealei).Thesepairshavebred
successfully
for at leastthree years.Additional observations
were madeon three pairsfrom
the westernUnited States(F. p. anatum),two pairsfrom northernQuebec,and one pair
eachfrom Alaska(F. p. tundrius),Spain(F. p. brookei),and the QueenCharlottes(F. p.
pealei). All thesepairsattemptedmating,and the femaleslaid eggs.Behavioral
observations
were madein 1972 and 1973 from April throughmid-Juneand in 1974 and 1975 from late
Januarythrough May.
Three pairsof Gyrfalconsfrom northernQuebecwere studiedfor two years.Only one
pair producedyoungin 1974 and 1975. Observations
weremadefrom earlyJanuarythrough
April. The Gyrfalconsand arcticPeregrines
(F. p. tundrius)wereplacedon a schedule
of
advancing
photoperiods,
asdescribed
by WeaverandCade(1974). Thisschedule
istimedfor
Peregrines
so that all subspecies
are roughlysynchronized
to beginegg-laying
at aboutthe
samedate. We made incidentalobservationsover a three-yearperiodon four to five produc-
tive pairsof PrairieFalconsandon two pairsof LannerFalcons,oneof whichhasproduced
multiplebroodsfor four consecutive
years.
There was no fixed scheduleof observationduring 1972 and 1973. In 1974 pairs were
observedregularlyfive days per week and periodicallyon the other days. Study was concentratedbetween dawn and 1100 hours,and, twice per week, for three to four hoursprior
to darkness.These are periodsof greatestactivity. Pairswith advancedphotoperiodwere
observedfrom the time the lights went on until 1100 hours. Additional observationswere
scatteredthroughout the day. Observationsin 1975 were made from "dawn" until 1100
hoursevery day. Additionalobservations
weremadeat other timesof day, with emphasison
the hours before darkness.
Throughout our studies.the falcons were housed in the Cornell BehavioralEcology
Building,Ithaca, New York. Pairswere kept togetherthroughoutthe year.Thisfacility was
equippedfor about 35 pairsof falconsand their young.Chamberdimensions
and fixtures
were describedby Weaverand Cade(1974). Visual exposureof the falconsto humanswas
minimal, and none of the falcons was tame enough to allow a human to enter its room
without becomingalarmed.Feedingwas accomplished
throughchutesat two levels.Dead
four-week-oldchickensand adultCoturnixQuail(Coturnixcoturnix)wereprovidedasfood.
Rooms had to be entered occasionallyto changewater baths and, when necessary,to
examine the health of a bird. Observationthrough one-way mirrorswas possiblefrom two
levelsin each breedingchamber.In addition to handwrittennotes, still photographyand
video-taperecordingswere used for the analysisof behavior.In both cases,pictureswere
Spring/Summer
1977 Wrege& Cade- Behavior
of Falcons
taken through the one-way mirrors without additional illumination.. Vocalizations were
recordedwith a NagraIIIB recorderand an Altec microphoneand were analyzedon a Kay
ElectricSonographwith a wide-bandfilter.
Results
Behaviorduringthe first successful
breedingseasonfor a pair hasbeendeemphasized
in
our analysisof results.Duringthe first year, reproductivebehavioris often contractedinto a
shortand accelerated
courtshipperiod.Behaviorpatternsappearidenticalto thoseof experiencedpairs,but their frequencies
and seasonal
ontogenyaredifferent.CaptivePeregrines
showno subspecificdifferences
eitherin the patternsof behavioror in its ontogeny;how-
ever,sample
sizesaretoo smallto be certainthereareno average
differences
thatmightbe
revealedby statisticallytreatablesamples.
Nearlyall behaviorpatternsdescribed
hereinaredisplays
in that theyaresignals
that have
becomespecialized
for communication
(Brown1975).Thesequencing
of displays
intolarger
recognizableunits, with their own specializedsignal,is very common in animals. In this
papertheseunits havebeen givendescriptivenameswhich are capitalizedand calledDis-
plays.The termsdisplayandpostureare usedinterchangeably
for behaviorpatternsthat
makeup or occurindependently
of the morecomplexDisplayunits.The namesof vocalizations are alsocapitalized.
Descriptions
andDefinitions
of Behavior
in Peregrine
Falcons.Amongthe manybehavior
patternsof captivePeregrineFalcons,13 Displaysanda few otherbehaviors
areparticularly
usefulin describing
the pairrelationship
andthe seasonal
ontogenyof reproductive
behavior.
Many of thesehave been at leastpartly described(e.g., for wild Peregrines-Cade1960,
Fischer1968, Nelson1970; for captivePeregrines-Fyfe1972, Nelsonand Campbell1973,
1974, Weaverand Cade 1974). In this sectionwe providea descriptivesketchof these
Displaysin captivePeregrines
with somedetail(l) on thosenot previously
described
or (2)
in caseswhenthe behaviorof our pairswassignificantly
differentfrom published
descriptions.
1. Head-LowBowDisplay.Fourvariations
of thisDisplayoccurin contextsrangingfrom
anti-aggressive
through
mildlyaggressive.
Theyareexhibited
by eithersexin response
to
movement
or close
proximity
of themate.Thebasicmodeof thisDisplay
isnonaggressive,
andthe postures
it includesarecharacteristic
of manysequences
preliminary
to andduring
closemutual interactions.Thesecharacteristics
includeholdingthe headbelow the body
plane,beak directedaway from the mate andusuallytowardthe substrate,
andgenerally
sleekedplumage.
There are horizontaland verticalformsof this Display.The HorizontalHead-LowBow
involves
crouching
in a horizontal
bodyposition,the headbentat almost90ø to the body
plane and the beak often contactingthe substrate.The Vertical Head-Low Bow is a less
intenseform, givenwith the body in a normal perchingposition,but with the head de-
pressed.Body positionsintermediatebetweenverticaland horizontalare frequentlyobserved,and thereis completeintergradation
in the amountthe headisbowed(fig. la). Either
form of thisDisplaymay involvevigorous
bowingup anddownfromthehead-lowposition
to a normalposturewith the headabovethe body plane(seeNelsonandCampbell1973).
Often the Head-LowBowis maintainedwithoutvigorousbowing,especially
whenin close
proximityto the mate. Severalvocalizations
may be givenduringthe Display,includingthe
EechipandWhinevocalizations
(figs.2a and2b). It is alsofrequently
unaccompanied
by
calling.Mueller(1971) hasdescribed
similardisplaysin the AmericanKestrel(FalcoSparverius).
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RAPTOR RESEARCH
Vol. 11, No. 1/2
A third variationof this Displayis the ExtremeHead-LowBow (fig. lb). Its functionis
anti-aggressive,
and it appearsto be a very intenseform of the HorizontalHead-LowBow
describedabove.During this Displaythe body is tipped far forward so that the tail is very
high and in line with the body. Slightbowingmay be included,andthe l•echipvocalization
is frequentlygiven.This Displayapparentlyinvolvesa mixture of two motivationalstatesfear and copulation.It is givenmostoften by the femaleduringMutualLedgeDisplays(see
below) and especiallyjust before aborted attemptsto copulate.The generalform of this
Displayis, in fact, quitesimilarto the female'sCopulation
Solicitation
Display(seebelow).
The AgonisticHead-Low Bow is a fourth variationof the more generalHead-LowBow
Display.ThisDisplayincludesthe "deepforwardbow" described
by Nelsonand Campbell
(1973). It is givenby either sex in agonisticsituations,and by the male in precopulatory
behavior.Featherson the head, especiallyon the sides,may be flared out, and featherson
the shouldersare often raised. The head is held below the body plane, but the beak is
frequentlydirectedat the mate. Horizontaland verticalbody posturesare used,the latter
especiallyby the maleduringprecopulatorydisplay(seeHitched-Wing
Display).l•echipor
Chittervocalizations
sometimes
accompanythis Display(figs.2a and2c).
2. Individual Ledge Displays. Individualledgedisplaysare givenby the male or female
(Male LedgeDisplayor FemaleLedgeDisplay,respectively)
aloneon a prospective
nestledge.They are usuallycenteredon a scrape(a shallowdepression
madein the substrate).
Basicbehaviorpatternswere identicalin all subspecies
studied.Major differencesinvolved
vocalpeculiarities,
whichmayrepresent
individualvariationmorethansubspecific
difference.
a. Male LedgeDisplay. The scrapeis approachedin a horizontalhead-lowpostureaccompaniedby a continualEechipvocalization(figs. 1a and 2a). Whensexualmotivationis
high, a "high step" or "tippy-toe" gait is usedand producesa side-to-side
swagger(also
describedby Nelson and Campbell 1973 in another context). The horizontal head-low
postureis maintainedduringintenseactivity at the scrape,and a completeEechipvocalization is givenrepeatedly.Pausesbeginafter five to ten seconds,
duringwhichthe malelooks
toward the female. At any time, movementby the femaleis likely to elicit renewedintense
display,andher reactiondetermines
the durationof display.At low intensitythe malemay
becomerelaxed,and the vocalizationthen is usuallyan incompletevariationof the Eechip.
b. Female LedgeDisplay. FemaleDisplaydiffersfrom male Displayin severalways.In
generalit is lessintenseandis sometimes
difficultto distinguish
from nondisplay
activityon
the nest-ledge.The posturesare less distinctiveand more variable.Approachis usually
entirelyhorizontal(i.e., head,body, andtail all in oneplane)or with a slightloweringof the
head. A complete Eechip vocalizationis given. The female turns aroundin the scrape,
mandibulates
debrison the ledge,and scrapes
frequently(seebelow).Pausesto look at the
maleareinfrequent.
Female
Ledge
Displays
oftenchange
intoapeparent
noncommunicative
activity.
3. Mutual LedgeDisplay. Simultaneousactivity by both sexeson the nest-ledge,
usually
centeredon a scrape,characterizes
this Display.The mostintenseportionoccursjust asboth
birds arrive at the scrape,eachin the horizontalhead-lowposturewith beakscloseto the
substrate,vigorouslyEechipping.Movementsof eachsexrelativeto the other and the characteristic pausesthat occurduringthe Displayhavebeen describedby Nelsonand Campbell
(1973, 1974).
Interactions with movements,postures,and vocalizationsidentical to those in Mutual
LedgeDisplaysmay occurinfrequentlyon perchesother than the nest-ledge.
4. Billing. Billingis often seenduringthe longerMutual LedgeDisplays,and occasionally
when the pair is perchingvery closetogether.Billinginvolvestwistingthe head sideways,
Spring/Summer1977
Wrege & Cade- Behaviorof Falcons
ß-,;;'.,,...'
:'.••.-'%
. ,,
-•..
-;½.•.•,
• _;.ß.. •...
Figure1. Courtshipposturescommonto four species
of largefalcons.(a) MalePeregrine
approaching
the scrapein a HorizontalHead-LowBow. (b) Peregrines
duringa partly
aggressive
interaction;female on left in Extreme Head-LowBow; male in a transitional
postureshowingsomeaspectsof the Hitched-WingDisplay.
Spring/Summer1977
Wrege & Cade- Behaviorof Falcons
' *' -*X* .e74.,
:,,.--.
-•
•,,,.•-
ß,*;',,*'•---*:
* • ',,::,
,'*• i' ,.',..x,,......,--
ß-•'..,•
:
.,...,-
..
(e) Gyrfalcons
asthe malefliesto mount;femalein CopulationSolicitationposture.
(f) Gyrfalconcopulation;malein Curve-Neckposture.
Spring/Summer1977
Wrege & Cade- Behaviorof Falcons
(e) Gyrfalconsasthe malefliesto mount;femalein CopulationSolicitationposture.
(f) Gyrfalconcopulation;malein Curve-Neckposture.
8
RAPTORRESEARCH
Vol. 11, No. 1/2
Spring/Summer1977
Wrege & Cade- Behaviorof Falcons
especiallyby the female,and nibblingbetweenbeaks.The female'sheadis usuallyvery low
with her beak directed upward, while the male facesdownward.If billing occursduring a
Mutual Ledge Display, the normally loud Eechip vocalizationtends to diminishtoward
Peepingand quietfemaleChupping-incomp!ete
variationsof the Eechipsoundunit.
5. Scraping.Scrapingis exhibitedby either sex duringsolitaryactivity on a ledgeor as
part of an IndividualLedgeDisplay. There is somequestionas to whether the behavior
shouldbe considereda componentof Display.Duringscrapingthe body is cantedforward,
weight on the breast,beak frequently in the substrate,the tail relaxedand slopingtoward
the ledge.A shallowdepression,
the scrape,is madeby vigorousbackwardpushingwith the
feet. Thisbehavioroften occursin a serieswith a shift in positionbetweenboutsof scraping.
No vocalizationaccompanies
this behavior.With the exceptionof femalesfrom about five
days before laying, no "rocking" movementis made before settlingon the breast.The
rockingbehavioralwaysoccursas a falcon settlesonto eggsfor incubationand hasbeen
describedin detail by Nelson(1970) for wild falcons.The behaviorin captivebirds is
identical.
6. Food-Transfer Display. A common courtship Display involvesthe transfer of food
from one mate to the other, usuallymale to female. Either sex may initiate a transfer.The
female usesa Wail vocalizationor rarelya Whine,combinedwith a verticalhead4owposture
to solicit transfers when the male does not have food. If the male has food, the Wail and
Eechipvocalizations
are usedabout equallyby the female,often accompanied
by the Vertical Head-LowBow Display.
Male solicitation,whichelicitsthe female'sapproach,alwaysoccurswhenhe hasfood,
either spontaneously
or initiated by femaleintentionmovementsto engagein transfer.This
solicitationby the male is characterized
by a very sharpand clearEechipvocalization.The
male alternatesbetweena relaxedposture,with the head up, and a posturewith his head
down while he manipulatesor contactsthe prey item. This posture,with the headlow, does
not appearto be the nonaggressive
Head-LowBowDisplay.Transferfrom the femaleto the
male is not obviouslysolicited.
Prior to actual transfer,the male picksthe prey item up in hisbeak and standsvertically,
head up. The female maintainshead4ow postures,often horizontal, and both sexesgive
completeEechipvocalizations.
Nelson-and
Campbell(1974) havedescribed
variationson the
actualtransfersequenceand behaviorassociated
with incompleteDisplays.
7. Hitched-Wing
Display.Engagedin by both sexes,this Displayis especially
characteristic of the male throughoutthe reproductivecycle, developingas sexualmotivation reaches
its peak (Weaverand Cade1974). It is consistently
givenin flight to and from copulation
and duringmale precopulatorybehavior.This Displaycanbe dividedinto two forms,flying
and standing(figs. 1c and 1d). The latter is probablythe samebehaviorasthe SlowLanding
Displaydescribed
by NelsonandCampbell(1973, 1974).
During Hitched-Wingflight the wings are held high, with short wing-beatsmostly from
the wrist. The legsare well forward, and the tail is depressed
resultingin a slow-motion,
bouncingflight. Frequently the flight path involveslow approachto the perch with a
last-minuteboundaboveandthen straightdown onto the perch.No bounceoccurswhenthe
male flies to mount for copulation.
Standing
Hitched-Wing
Displays
occurbrieflyto moderately
long(2 sec.
onds)afterthe
male landson a perch, frequentlyin the context of Mutual LedgeDisplays.It is always
expressedprior to copulation.Most often the body postureis verticalto semihorizontal,
high on stiff legs.The head is low, and the wingsare hitched up high againstthe body to
form a deep,V-shapeddepression
alongthe back. Anothervariationincludesa horizontal
head4owposition,legsstiff andwingshitched.
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RAPTOR RESEARCH
Vol. 11, No. 1/2
The maleprecopulatorypostureis especiallyinterestingin its combinationof the sexually
motivated Hitched-WingDisplay and componentsof the AgonisticHead-Low Bow. The
body is vertical,wingshitchedand legsstiff. The headis lowerthanthe shoulders
with beak
directedat the female, which is usuallysolicitingcopulation(seebelow). Vigorousbowing,
frequentlywith a side-to-side
swing,is part of this Display.The Chitteris a frequentmale
vocalizationduringprecopulatorybehavior.
8. CopulationSolicitationDisplay. The female'smotivationto copulateis communicated
by a seriesof posturesandvocalizations,
partiallydescribedby NelsonandCampbell(1973).
Solicitation may begin with the Whine vocalization,concurrentwith or just precedinga
Vertical Head-Low Bow. This is usuallygiven when the male is at somedistance.Primary
solicitationwill follow if the male showsreaction.During primary solicitation,either following the verticalsolicitationjust described
or independently,
the femaleassumes
a horizontal
head4owposture.Againthe Whine vocalizationis given,the tail is closeto horizontal,panel
feathersare raised,and her orientationis usuallyperpendicularto the male. This phaseof
the CopulationSolicitationDisplaymay continuefor up to 30 seconds.
Justas the male
showsintention to mount, the female sleeksher panel feathers,crouchesand leansforward
slightly, and sometimesbeginsto move her tail up and to the side in preparationfor
copulation.
9. Copulation.Duringcopulationthe femaleis pitchedforward,makingan angleof about
45ø withrespect
to theperch.TheCopulation
Wailisgiventhroughout
(fig.2d). Asthemale
mounts,the femalespreadsher wingsout at the elbow about one-fourthopen.The taft, up
and to the side,may be partly spread.
The male flaps his wingsthroughoutcopulation,maintainingan uprightposturewith the
neck extendedand bent in a curve (fig. If). Usuallythe male givesone or two burstsof
the Chitter vocalizationjust before,during,and/orjust after mounting,and then Eechips
sporadically.
Someindividualsgiveburstsof Chitterthroughout.Towardthe endof copulation the male stopshis tail movements,pressinghis cloacaagainstthe female's.Rapid
wing-beatsaccompanythis tail-press.The femalemay spreadher tail partly at this time, and
the male departswith a Hitched-WingDisplaydirectly afterwards.
10. Threat Behavior. The two major Displays have been describedby Nelson and
Campbell(1973). The characteristic
posturefor HorizontalThreatis with tail, body,and
headall in a horizontalplane.The beakis directedat the mate,wingsslightlyextended,head
and body featherserect.In UprightThreat,the body is verticalwith mostfeatherserect.The
tail and wingsmay be spreadto varyingdegrees;
the beakis usuallyopen.
Comparisonof Peregrineand Gyrfalcon CourtshipBehavior
We presenthere detailsof only thoseaspectsof Gyrfalconbehaviorthat differ from the
Peregrinebehavioralreadydescribed.Most of the courtshipDisplaysand behaviorpatterns
of these two speciesare very similar and can be designatedby the samenames.Unless
otherwisestated,components
of the variousbehaviorpatternsin both species
are typifiedby
the descriptionfor PeregrineFalconsin the previoussection.
1. Head-LowPostures.Head-Lowposturesare exhibitedby both speciesin quite similar
contexts. Gyrfalconsshow Horizontal and Vertical Head-Low Bow Displays,and use
head-lowposturesduringall LedgeDisplaysand Food-Transferring.As in the caseof Peregrines,the accompanyingvocalizationsare somewhatvariable;most often a Whine or brief
Chitter is given for the Bow Displays,and Chuppingoccursduring LedgeDisplaysand
Food-Transferring(fig. 3).
The frequencywith which theseDisplaysand posturesare exhibitedis the majordifference between the two species.In Gyrfalconsthere is very little intermediatevariationbe-
Spring/Summer
1977 Wrege& Cade- Behaviorof Falcons
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RAPTOR RESEARCH
Vol. 11, No. 1/2
tween the verticaland horizontalforms of the head-lowpostures.The thresholdfor assuming the moreintensehorizontalhead-lowpostureis highin maleGyrfalcons,andfemalesuse
the Head-Low Bow Displaymuch lessfrequently than Peregrinefemales.Neither sex of
Gyrfalconshowsthe vigorousbowingso conspicuous
in Peregrines.One of the most frequent contextselicitingHead-LowBowDisplaysin Peregrines
is the approachof the mateor
intention movementsof approach.Use of the Displayin this contextwasmore frequentfor
malesthan for femalesin both species.The Gyrfalconswere lessintenseabout the interaction, and intention movementsrarely elicited display.Frequentlythe male did not display
until the femalewasactuallylandi•.gon his perch.
In our Gyrfalconpairs there was no behaviorpattern strictly parallelto the Agonistic
Head-LowBow observedin Peregrines.
FemaleCopulationSolicitationhassomecharacteristics in commonwith the AgonisticBow, but, in general,orientationaway from the maleis
necessary
for mounting.Behaviorcomparable
to the ExtremeHead-LowBowwasnot observedin the Gyrfalcons.
2. IndividualLedgeDisplays.The contextsin which theseDisplaysoccurare identicalin
both species.Visual contactwith the mate is very important,reactionof the mate beinga
determinantof the Displayintensityand duration.
The Male Ledge Displaysof Gyrfalconsare nearly identicalto thoseof Peregrines.The
vocalizationin Gyrfalconsis alwaysa completeChup as comparedwith a tendencyof the
Peregrinevocalizationto degenerate.The reasonmay be that the basicGyrfalconvocalunit
is a singlesyllable,whereasthe Peregrine's
is complex.Thereis a possibilitythat Male I.edge
Displaysare more frequentandmorevocalin Gyrfalcons,but the samplesizeis too smallto
be certain.
Female LedgeDisplaysin both speciesshow similardifferencesfrom Male I.edgeDisplays.Theseincludelesswell-definedposturesand a markedtendencyto changeinto noncommunicativeactivity. The female Gyrfalconmay more consistentlymaintain a head-low
postureearly in the Display.
3. Mutual LedgeDisplay. Althoughthe functionsand generalcharacteristics
of the Mutual LedgeDisplayaresimilarin Peregrines
andGyrfalcons,differences
in movement,duration,
and vocalization
areconspicuous.
In both species
the Displayis primarilyorganizedarounda
potential nest-scrape
and is frequentlyprecededby a Male LedgeDisplay.In Peregrines
as
well as Gyrfalcons,as the Mutual Displaybegins,both sexesare in a horizontalhead-low
posture,
In PeregrineFalconsthere is considerable
movementby one or both sexesaroundthe
scrapeas well as pausesin the Displayfollowed by renewedvigorousactivity. In some
Peregrinepairsthe female is as likely as the male to terminatethe Displayby leavingthe
scrape.By contrast,Gyrfalconstend to maintainstationarypositionsduringthe Mutual
I.edgeDisplayand rarelypause.It wasextremelyrarefor our femaleGyrfalconto terminate
a Mutual Display.Our male Gyrfalconusuallyterminatedthe interactionafter only five to
ten seconds,
resultingin Displaydurationsshorterthan wasusualin Peregrines.
The greatestapparentdifferencebetweenthe vocalizations
of GyrfalconsandPeregrines
is changein repetitiveness
of vocalizationsduring the Display, rather than the obvious
differencesin the basicsoundunits. Male and femalePeregrines
givean Eechipvocalization
with somevariabilitydependingon the intensity.In addition,Peregrines
showconsiderable
variabilityin the regularitywith which successive
unitsof the vocalizations
are given.By
contrast,male Gyrfalconsgive very regularChup vocalizations
throughoutthe Display.In
the female Gyrfalconthere is alwaysa distinctchangein vocalizationasthe maleleavesher
aloneat the nest-scrape.
Duringmostof the interactionshegivesa seriesof veryfastChup
Spring/Summer1977
Wrege & Cade- Behaviorof Falcons
13
units,but theseincrease
in speedto a Chatterasthemaledeparts(fig.3d). Whenthefemale
of either speciesremainsin the scrapeafter the malehasgone,vocalizationdiminishes,
becoming
sporadic,
soft,and,in Peregrines,
incomplete.
The Gyrfalcons
gaveMutualLedge
Displaysonly at the nest-scrape.
Peregrines
occasionally
exhibitedidenticalinteractionsat
other locations,especially
early in the courtshipperiod.Billingwasnot observedduring
courtshipin the successful
pairof Gyrfalcons,but it hasbeenseenin otherpairs.
4. Scraping.The major characteristics
of this behaviorpatternare identicalin the two
species.
The conspicuous
"rocking"movement
of Peregrines
astheysettleon eggs(Nelson
1970) are presentin mostscraping
boutsby captiveGyrfalcons.Thismovementwasobservedin malesand femalesbeginningin January,when scrapingactivityfirst began.It was
observed
duringPeregrine
courtship
only asthe femaledid it a few daysbeforelaying.
5. Food-Transferring.The behaviorpatternsthat compriseFood-Transferringarevery differentin Peregrines
andGyrfalconsalthoughthe functionof the Displayappears
to bethe same
in both species.
It is an importantcourtshipinteraction,expressed
somewhatmorefrequently in the captiveGyrfalconsthan in the Peregrines.
The frequencyof female-to-male
transfer
waslow in both species,
but unlikePeregrines
the maleGyrfalconrarely took food to the
female'sperchfor transferring.
It wasmoreusualfor him to prolongsolicitation,waitingfor
the femaleto approachfor the transfer.
Comparisons
of vocalizationare similarto thoseof the Mutual LedgeDisplay.Both sexes
of Peregrinegive a completeEechipvocalization,and the male givesespeciallyclearand
sharpEechipswhensolicitinga transfer.GyrfalconsgiveChupvocalizations
throughmostof
the interaction.The female increases
the speedof repetition to a Chatter as the transfer
occurs.
The posturesof both sexesare different in the two species.In contrastto the upright
postureof a malePeregrine,the maleGyrfalconmaintainsa verticalto horizontalHead-Low
Bow while solicitinga Food-Transfer.The male Gyrfalconoccasionallylooks up at the
female during solicitation,but on her approachhe picks up the food in his beak and
maintainsa head-lowpostureuntil the transferis completeor the female losesinterest.
Female Gyrfalconsapproachin an entirely horizontal postureor slightlyhead-low.This
mildly aggressive
postureis maintainedduring the actual transfer and contrastswith the
conspicuous
head-lowpostureof femalePeregrines
throughoutthe Food-Transfer
sequence.
Malesof both speciestend to leavethe areaof transferimmediatelyafter the interactionis
complete,but the tendencyis particularlypronouncedin Gyrfalcons.Food-Transfersolicitation by the femaleis similarin the two species.As is usual,the Gyrfalconstend to be more
vocal, includinga nearly continualvocal responseby the female from the onset of male
solicitation until the actual transfer.
6. Male PrecopulatoryDisplay. Male Peregrinesand Gyrfalconshave distinctivepostures
usedduringprecopulatory
sequences.
Theseare the Hitched-Wing
andCurve-Neck
Displays,
respectively.The frequencywith which theseDisplaysappearis very different in the two
species.
The Hitched-Wing
Displayis first seenearlyin courtshipand appearsto functionasa
generalsignal.In Gyrfalconsthe Curve-NeckDisplaywas observedonly whenthe malewas
motivatedto copulateor just prior to a copulationattempt. Because
of thisspecialized
use
of the Curve-Neck
Displayin Gyrfalcons,
it alwayselicitedsomefemaleresponse.
A closecomparisonis possiblebetween PeregrineprecopulatoryHitched-WingDisplay
andGyrfalconCurve-Neck
Display.DuringtheseDisplaysthebodyis drawnup to maximum
height, and the plumageis sleeked.Gyrfalconsdirect the beak away from the female, and
Peregrinesoften direct the beak toward the female. Body posturesaccentuatethe head
positionin both species.
Gyrfalconsextendand bend the neckinto an invertedU shape;
14
RAPTOR RESEARCH
Vol. 11, No. 1/2
Peregrineshave their wingshitched over their backs,accentuatingthe head-lowposture.
When the female is very close,nonaggressive
posturesare added to or replacethe above
postures,at leastin Gyrfalcons.MaleGyrfalconseitherassume
a head-lowverticalpostureat
high intensities,or turn perpendicularly
to the female,maintainingthe Curve-NeckDisplay.
Whenin closeproximity to the female,malePeregrines
presenta profile,althoughthe body
may still be orientedtowardthe female.At higherintensities
malePeregrines
will frequently
drop to a horizontalbody posture,with Hitched-Wingand the AgonisticHead-Lowposition.
Males of both speciesuse vocalizationsduring theseDisplaysthat are alsousedduring
clearly aggressive
interactions.The male Peregrinefrequentlyemits the Chitter vocalization
(describedby Cade1960, Nelson1970)just prior to mountingattempts.This vocalization
has also been heard when the female was trying to pull food away, duringexchangesfor
incubation, and while in Horizontal and Upright Threat. Male Gyrfalcon vocalizationsjust
prior to mountingwere difficult to resolvebehindthe overridingvocalizationof the female.
Recorded segmentsthat have been analyzed appear similar to the agonisticvocalizations
givenduringHorizontal and Upright Threat.
7. Copulation Solicitation. Some posturalaspectsof female Solicitationfor Copulation
differ betweenspecies,but the Displayis similarin its progression
and vocalcharacteristics.
Generally there seemsto be more of an agonisticcomponentto Gyrfalcon solicitation.
Althoughboth femalesposturehorizontallyduringprimarysolicitation,the femaleGyrfalcon often approaches
head-onin an entirelyhorizontalposture(i.e., components
of horizontal threatening,fig. l e), and the Peregrinefemale is stationary,usuallyorientedeither
perpendicularto or away from the male. Female solicitationin Peregrines
is distinctly
head-low. The female Gyrfalcon does turn perpendicularlywhen closeto the male, and
copulationproceededin our pair whenthis orientationwasmaintained.In both species
an
initial solicitationwas sometimesmade from a verticalhead-lowposture,usuallyat some
distance from the male.
8. Copulation.FemalePeregrinesandGyrfalconshavedistinctiveWail vocalizations
given
onlyduringcopulation.
Theirbodiesaretippedforwardto an angleof 45ø, legsstiffand
head in the body plane. The vocalizationemitted by the male is variableeven for an
individual,but is usuallygivenin bursts.This vocalizationin Gyrfalconsappearsto be the
sameas duringthe precopulatorysequence,
althoughclearSOhographs
couldnot be made.
The copulationpostureof malesis identicalin Gyrfalconsand Peregrines.
It is a vertical
posturewith the Curve-Neck
headposition(fig. If). In Peregrines,
at least,the talonsare
balledup into a loosefist,weighton thetarsi(Nelson1970;seealsoMueller1970).The
talon positionis difficult to seein Gyrfalconsowingto the denseplumage,
but theyappear
to be balledup also;sometimes
the male'shalluxappearsto be lockedunderthe female's
humerus.
9• Aggressive
Behavior.
Thebehaviors
in thiscategory
aresimilar
in thetwospecies.
Well-adjusted
pairsrarely showedany agonisticbehavior,and UprightThreatwasnot observedexceptin newand/orincompatible
pairs.
Commentson the CourtshipBehavior of Lanner and Prairie Falcons
AlthoughLannerand PrairieFalconshavenot beensubjectedto the samedetailedobservation as our Peregrinesand Gyrfalcons,we have enoughincidentalobservations
to know
that all basicDisplaysdiscussed
in the previoussectionareusedby thesespecies
too. In most
casesthe forms of their Displaysand vocalizationsbear strikingresemblances
to thoseof
Gyrfalconsand servefurther to emphasizethe closephylogeneticties amongtheseforms,
Spring/Summer1977
Wrege & Cade- Behaviorof Falcons
15
whichare usuallyalliedin a separate
subgenus
fromthePeregrine.
Thedisplays
andvocalizationsof PrairieFalcons
and,in particular,
Lanners
aremoresubdued
andlessconspicuous
thanthe vigorous
andlouddisplays
of the Gyrfalcons,
but otherwise
therearefewqualitative differences.One exceptionis the Chuppingcall of the PrairieFalcon,which is more
similar
to thePeregrine's
"Eechip"
thanto theGyrfalcon's
"Chup"
(fig.4a).TheCurveNeck
Displays
ofthemales
ofallthree
species
priortocopulation
arestrikingly
sinilar
and
stand
in maked
contrast
to theHitched-Wing
Display
of themalePeregrine.
Onecharacteristic
ofPrairie
Falcons
thatisdifferent
fromtheothers
isthehigh
degree
offemale
aggression,
which
often
erupts
intoovert
attack
onthemale
during
theearly
stages
ofpairing.
Asa
consequence,
Head-Low
Bows
andotherforms
of agonistic
display
occur
morefrequently
andoccupy
a greater
portionof thetotalcourting
periodthanin thecaseof theother
species.
SeasonalOntogenyof ReproductiveBehavior
Owing to the individual variability in seasonaldevelopmentand the small sampleof
pairedGyrfalcons,this discussion
of seasonal
ontogenyis limited to Peregrines.
Initial courtshipinteractionswere observedearlierin eachsuccessive
year of breedingfor
at least the first three years. Egg-layingalsotended to begin somewhatearlieralthoughthe
courtshipperiod still lengthenedeach year. The earlier onset of copulationwith respectto
laying dates was especiallyconsistent;all pairs showedthis progression
(table 1). There is
some evidenceto suggestthat the seasonalontogenyof behaviorbecomesstabilizedafter
severalyears.The three experiencedpairs(two breedingfor the third time, one for the third
and fourth times) showedstrikingsimilaritiesin courtshipdevelopment.
All pairsinitiated
courtshipat about the sametime, began copulation within one week of each other, and
beganto lay eggswithin a periodof ten days.The developmentof courtshipoutlinedbelow
usesthe temporal progressioncharacteristicof the pairs in their third or fourth breeding
season(fig. 5). The actualdatesusedin this sectionare specificto our particularenvironmental conditions.They are expected.to vary dependingon local weather conditions,latitude, and photoperiodmanipulation.
A gradualincreasein activity on the nest-ledgeby both sexesis the first indicationthat
courtshipis beginning.Ledgebehavioris most conspicuous
in the male, beginningin early
January. He displaysat severalscrapes,often on more than one ledge. This pattern of
maintaininga numberof scrapesis exaggeratedin the young pairsbreedingfor the first time.
Duringthis early periodboth sexesfrequentlyuseHead-LowBow Displaysor Mild Threat
whenapproachedcloselyor suddenly.
Toward the end of January,Mutual LedgeDisplaysand Food-Transferring
beginto
developsimultaneously.The female showsinterestin Male LedgeDisplaysand alsohesitates
before gettingfood when it is first introduced.This hesitationpermitsthe maleinitial access
to the prey (seeWilloughbyand Cade1964). Solicitationfor Food-Transferring
is displayed
repeatedlyby the male althoughif the femaleapproaches,
he tendsto moveaway, resuming
solicitationfrom a new perch.
Male Hitched-Wingflying becomesapparentabout one month after the onset of courtship. For an additional two or more weeksthe female respondsdirectly to male HitchedWing flights over her or close by. This reaction is usually the Vertical Head-Low Bow
accompanied
by either Eechipor sometimesthe Whine vocalization.Apparentlythe female
becomeshabituatedto theseDisplays,as the male usesHitched-WingDisplaysfor almostall
RAPTOR
16
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Vol. 11, No. 1/2
Spring/Summer1977
Wrege & Cade- Behaviorof Falcons
17
Table 1
The relationshipof breedingexperienceto the start
of copulationduringcourtship.
Subspecies
and
year of breeding
Dateof first
observedcopulation
Daysprior
to first egg
F. p. tundrius
pair CC:
first year (1972)
third year (1974)
fourth year (1975)
pair CH:
first year (1973)
secondyear (1974)
third year (1975)
pair U8:
first year (1975)
F. p. pealei*
pair MP:
first year (1973)
secondyear (1974)
third year (1975)
April 10
2
March 5
15
March 3
23
April 8
5
March 7
10
February 26
24
May 16
April 1
March 22
March 3
0
2
4
15
* Thispairbredsuccessfully
for two yearsbefore1973 at anotherbreedingfacility.
18
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UOIAVH38
Vol. 11, No. 1/2
Spring/Summer1977 Wrege & Cade- Behaviorof Falcons
19
movementsacrossthe room, until the end of incubation.Youngmalesare not asconsistent
in usingthe Hitched-WingDisplay. Femalereactionto this activity steadilydecreases,
and
late in the courtshipperiodher reactioninitiatesstepstoward CopulationSolicitation.The
Whine vocalizationis almostalwaysusedin thesesituations.
For a period of about three weeks in February, Mutual Ledge Displaysand FoodTransferringdevelopinto frequent interactions,and Head-Low Bow Displaysbecomeless
frequent.Matingbehavior,includingfemalesolicitationfor coptdationand maleprecopulatury posturing,is first observedin late Februaryto earlyMarch,abouteightweeksafter the
onset of courtship.The femalesperform CopulationSolicitationDisplaysfor a variable
lengthof time beforethe malescompletethe sequence
by mounting,usuallyin lessthan two
days.In all casesthe early matingbehaviorwasidenticalto the precoptdatory
sequences
observed
later in the year. TheseinvolvefemaleCopulationSolicitationwith bowing,alternatingwith the male Vertical Head-LowBow or, moreoften, the precopulatoryHitched.
Wing.In both sexesthe bowingis vigorous:a fast,jerky movementto the bow position,then
up to the startingposture.
Initial CopulationSolicitationis observedaboutthreeweeksbeforelaying.At first many
copulationsequences
are incomplete,and successful
copulationsare short,averaging
five to
six secondsdependingon the pair.Two to threeweeksbeforelaying,copulationis alreadya
regularinteraction,with a durationof eightto ten seconds,
occurring
at a frequencyof two
to threecopulationsper hour duringthe mostactiveperiod(first hour of light). Oneweek
beforelaying,copulation
increases
to a frequencyof threeto fourperhourduringthefirst
hourof light.Copulation
continues
in mostpairsuntil the third eggis laid.Although_
coptdation
wasnot observed
in somepairsafterthe second
eggwaslaidandveryfewpairs
coptdatedafter the third egg,full clutcheswereconsistently
fertile. A suddenincreasein the
frequencyof coptdation
occurred
on the daythat the second
eggwaslaid,eithera few hours
prior to, or after, laying.
The male showslittle motivationto incubatethe first eggandtendsto continueMale
LedgeDisplaysaroundthe egg,sometimes
movingthe eggout of the nest-scrape
andthen
displayingin the empty space.The femaleincubatesimmediatelyif weatherconditions
requireit althoughmoreoften sheordystands
overthe egganddoesnot begincontinual
incubationuntil the secondor third egg.
Discussion
1. Limitations of Studying Captives.When behavioraldata are basedon the study of
captiveanimals,extreme cautionis necessary
in extendingconclusions
to includewild situations or evenother captiveenvironments.In addition to other factors,the frequencywith
which certainbehaviorpatternsoccur can be very different in captiveanimals.There is also
the temptation to consideras normal the behaviorof captiveanimalsthat successfully
reproduce.Such an assumptioncan be misleading.Highly unnaturalpair-bondsand behavioraldevelopmentmay still resultin fertilization,as in the caseof interspecificcrosses
(e.g.,MorrisandStevens1971).
One factor that might contributeto suchabnormalityis the stimulus-deprived
environment experiencedby captiveanimalsandthe resultantresponsiveness
to suboptimalstimuli.
A secondbias may come from exaggerationof individualdifferencesin behavior.Enforced
pairingand limited or incompleteenvironmentsaccentuateindividualvariationsin behavior
20
RAPTOR RESEARCH
Vol. 11, No. !/2
that may or may not be significantin nature. Evaluating the importance of these behavioral differencescan be helped by determiningthe causesof their expressionin the
captivesituation.
The interpretationof behaviorin captivityis dependent
on studyof the samebehaviorin
nature, not viceversa.The only exceptionmightbe studiesof organisms
for whichthe entire
rangeof environments
experienced
by the species
withinthe cycleof behaviorunderstudy
can be containedor accuratelysimulatedin the study chambe_r:
The captiveenvironmentof our falconscannotsatisfytheserequirements.The breeding
chambersprovidea minimal environment,satisfactoryin that successful
breedingcanoccur.
The behaviorof wild North Americanspeciesof Falco hasbeen studiedto varyingdegrees.
PeregrineFalcons,Prairie Falcons,and the AmericanKestrel(Falco sparverius)are best
known, and the behaviorof Gyrfalconsisjust beginningto be described.Very little is known
of the Lanner Falcon. Most of thesestudiesincludeonly incidentalbehavioralobservations.
A few haveconsidered
in moredetailthe repertoireandtemporalpatterningof reproductive
behavior(e.g., for AmericanKestrels,Cade1955;for Peregrines,
Fischer1968, Nelson1970,
Fyfe 1972,Wregeunpubl.;for Gyrfalcons,
Platt 1976).Thusa smallliteratureexistswith
whichto gaugethe behaviorpatternsof captivefalcons.
Severalbasicdifferencesare apparentwhen comparingthe behaviorof captivefalcons
with what is known of their behaviorin the wild. The closeproximity of the captivepair and
the limitedspacehaveresultedin an emphasis
on certainDisplays,
whileothersare
deletedfrom the repertoire.Food-Transferring
andespeciallyLedgeDisplaysare muchmore
frequent in captivePeregrines-probably
becauseof the almostconstantlyavailablestimuli
that elicit the behaviorpatterns. It is likely that other behaviorsvary considerablyin frequencywhen comparedto wild falcons.SomecourtshipDisplaysobservedin naturerequire
considerablespace,suchas, territorial flight Displays,mutual defenseDisplays,and others
(see Cade 1960, Nelson1970). Thesebehaviorpatternsare not observedin captivity,althoughsomeothersmay be modificationsof them.
The contextual useof certainvocalizationsis different in captiveand wild falcons.Evidence suggests
that many vocalizationsof falconscommunicateintensityof motivation,as
opposedto directionof motivationto perform a specificaction. The degreeto which this
might be true in wild falconshas not been determined.The closeproximity of matesin
captivity makesunnecessary
vocalizationsthat may function in the wild as long-distance
signals,and anothervocalizationis substituted(Platt pets. comm., Wregeunpubl.) For
example,a wild male Peregrinewill often solicitFood-Transferring
with a Wail vocalization
ashe returnsto the eyriesite.As the femaleapproaches
for the transfer,he beginsto Eechip.
In captivitythe male solicitsa transferonly with the Eechipvocalization,which continues
until the Displayis complete.
Individual behavioral characteristicsof captive falcons greatly influence reproductive
success.
Continuedcloseproximity and the difficulty of avoidinginteractionsare probably
contributingfactors.Observations
of wild Peregrines
and Gyrfalconsindicatethat malesmay
spendconsiderable
time awayfrom the females,eitherhuntingor perching
out of sight.No
data are available to determine whether individual differences in behavior affect choice of
mateandbreedingsuccess
in wild falcons,but suchinfluenceseemslikely (Cade1960).
2. TheFunctionof Displaysin Pair-FormationandPair-Bonding.
The Displaysandvocalizations of Peregrines,Gyrfalcons, Prairie Falcons,and Lanner Falcons can be divided
roughly into three groups.First, nonaggressive
behaviorpatternsincludethe following: All
the head-lowpostures;Head-LowBow Displays(excludingthe agonisticform); LedgeDisplays; Food-Transferring;
CopulationSolicitationWhine;the Eechipvocalizationand Ex-
Spring/Summer1977
Wrege & Cade- Behaviorof Falcons
21.
tremeHead-LowBowin Peregrines;
ChupandChattervocalizations
in Gyrfalcons;
Kuduchip
in Prairie Falcons; and similar vocalizations in Lanner Falcons. These behaviors can be
further subdividedinto anti-aggressive
or appeasementand approach-eliciting
behaviors,
accordingto the type of interaction.The usesof nonaggressive
displaysin thesetwo forms
are discussed
in the sectionon the pair-bondand femaledominance.
A secondgroupof behaviorpatternsincludesthe maleprecopulatory
postures
in each
species,female CopulationSolicitationin Gyrfalcons,and the Wail vocalizations
in each
species.It is difficult to labelthesebehaviorsas eitheraggressive
or nonaggressive,
In each
casethere appearto be aggressive
components.
Finally, the remainingbehaviorpatternscan be groupedas at least partly aggressive.
Theseare the AgonisticHead-LowBow Displaysin Peregrines,
any all-horizontal
posture,
Chitter vocalizations,and the Upright and Horizontal Threat Displays.Of course,none of
the threegroupsof behaviorpatternsis rigid.In eachspecies
(andin somemorethanothers)
postures
andespecially
vocalizations
tend to intergrade
depending
uponchanges
in motivation.
Many courtshipdisplaysin falconsappearto integratethe pair and to easeco-inhabitation
of a limited space.The effectiveness
of a displayand its frequencyare probablyrelatedto
the degreeof sexualreadiness
of the mates.Male LedgeDisplaysstimulatereproductive
behavior
in females
andinitiatepairintegration.
MutualLedgeDisplays,
Billing,andFoodTransferringall resultin close,nonaggressive
interactions.This doesnot imply that aggressionand fear are neverseenaspart of the Displays;rather,it meansthat the outcomeof the
interactionis nonaggressive
andhelpsto form a pair-bond.
Studiesof captiveandwild Kestrelssuggest
that copulation
mayfunctionin pairintegra-
tion very earlyin the reproductive
season
(Willoughby
andCade1964).Fischer(1968)
mentionscopulationasthe first interactionof the season
in somewild Peregrine
pairs,and
Fyfe (1972) has describedcopulationin Prairie Falconson the first day the pair was
togetherat the eyrie. This association
of copulationand initial pair-bondingwasnot observedin the four captivespecies
of Falco studiedhere.Established
pairsinitiatedcopulation
very early relativeto egg-laying,
but all other courtshippatternswerewell established
at the
time of first copulation.This patternof pair-bondinitiation,involvingconsiderable
courtshipactivitypreceding
first copulation,wasalsoobserved
in captivePeregrines
introducedto
oneanotherabruptlyat the startof thebreedingseason
(NelsonandCampbell1973, 1974).
Nonetheless,
copulationprobablydoesfunctionimportantlyin strengthening
the pair-bond.
Copulationin the Gyrfalconpair continuedfor 39 daysbeforeegg-laying,
progressing
from
very short attemptsthat wereterminatedby femaleaggression
throughfull-lengthcopulationsbeginnin;20 daysbeforelaying.
]'he Displaysin eachspecies
of Falco canbe characterized
by the samename and include
verysimilarpostures,
with the exception
of the Hitched-Wing
a•ndCurve-Neck
maleprecopulatoryDisplays.This likenessis suggestive
of a similarityin functionas well. Without
the analysis
of quantitative
data,it is difficultto determine
whetherimportantinterspecific
differences
existin the functionof various
Displays
in pair-bonding
andintegration.
At this
time, differences
are not apparent,evenwhen comparing
the phylogenetic
groupof Gyrfalcons,PrairieFalcons,and EannerFalconsto Peregrine
Falcons.The majordifferences
in
behaviorpatternsare of frequency,intensity,and materesponse.
Thesedifferences,
which
may be intraspecific
aswell asinterspecific,
appearrelatedto the natureof the pair-bond,
especially
the dominance
relationship
betweenthe sexes.
3. The Pair-Bondand FemaleDominance.The relationshipof male and femalein the
pair-bond
of falconshasbeeninterpreted
in differentways.In mostcases,
theinterpretation
22
RAPTOR RESEARCH
Vol. 11, No. 1/2
has been incidentalto an explanationof reversedsexualsize dimorphism,and this may
accountfor the lack of data specificallybearingon the relationship.The followingdiscussion
about the pair relationshipin captivefalconsdoesnot dependon any theoryconcerning
the
evolutionarypressures
causingsizedimorphism.Hagen(1942) suggested
that femaledominance was necessary
to avoid filicidal behaviorby the male. In most speciesof falcon, the
maleparticipates
to somedegreein incubationandcareof theyoung.Thereislittle evidence
to suggest
that a realthreatexists,and strongselective
pressure
to eliminatesucha tendency
in the male'sbehaviorwouldbe expected.Cade(1960) placedimportanceon the divisionof
labor that is very commonin Falco, suggesting
that femaledominancemight be necessaryto
maintain the male in his role as food provider.Amadon(1975) speculatedthat female
raptorsmay be "moresubmissive
or passive"
relativeto the male,at leastduringthe initial
pairinteractions.
To datetherehasbeenvirtuallyno studyof theseinitialinteractions
in any
raptor.A differentinterpretationof the pair-bondresultedfrom the studyof pealeiin the
QueenCharlotte Islandsby Nelson(1970). He suggested
that dominancewasrelatedto the
location of the interaction, with the male being dominant in aerial encounterswhere his
agility wouldbe an advantage,
and the femalebeingdominanton the nest-ledge
andduring
other closeinteractionswhereher sizewouldbe favorable.Observations
on a captivepair of
Arctic Peregrinesled to the interpretation that the male dominated the female in the
breedingchamber(Nelsonand Campbell1973).
There has been too little detailed behavioral analysis of wild falcons to determine the
dominancerelationshipduring reproduction.Our observations
of four speciesof captive
falconsindicatethat the femaleis dominantin all speciesand in pairsthat breedsuccess-
fully. Urnsuccessful
breedingcan often be correlatedwith eithervery dominantfemales,
inhibitingalmostany mutualbehavior;
with a lackof dominance
by eithersex;or, exceptionally, with a domineeringmale.
The relativefrequencyof aggressive
and nonaggressive
posturesin eachsexmay be used
as an indicatorof the pairrelationship.
The useof thesepostures
within ritualizedcourtship
Displaysas well as independentlyis important,and frequencies
appearpredictableon the
basisof the degreeof size dimorphismexhibited in the pair. A specificvalue for the
relationshipbetweensizedimorphismand the relativefrequencies
of thesebehaviors
would
not be foundin mostcaptivefalcons,for the followingreasons.
Differences
in the historyof
eachbird, primarilyin its handlingby humansand experiencewith conspecifics,
resultin a
largevariationin aggressiveness
that influencesthe frequencyof aggressive
andnonaggressive
behavior.We do feel, however,that a generalpattern is demonstrable.
The Peregrineis the mostdimorphicof the speciesstudiedand hasbeenobservedmore
intensivelythan the others.Possiblythe mostconspicuous
behaviorpatternthroughoutthe
reproductiveseason
is the Head-LowBow.Both sexesexhibitthis Display,and intergradation is nearlycontinuousbetweenthe lessintenseverticalform andthe extremehorizontal
form. In mostcases
this Displayappears
to be anti-aggressive
in meaning,
ratherthanapproach-eliciting.
Thisdistinction
is important.
An anti-aggressive
posture
is dearlyonethat
inhibitsaggression.
Approach-eliciting
postures
signalthe absence
of aggressive
motivationin
the sender.It is quite possiblefor the samepostureto take on either meaning,evenwhen
givenby the sameindividual.The meaningdependson the relativedominance
of the interactinganimalsin the contextof that interactionor in anticipationof the interaction.The
problemcomesin deciding
whichmeaning
a givenposturehasandin tryingto avoidshaping
the decisionon the basisof preconceptions.
We wouldlike to stressthat all thesecomparisonsare of relativefrequencies.
It isquiteevidentthat eachbird is intimidatingto the other,
Spring/Summer1977
Wrege & Cade- Behaviorof Falcons
23
andboth sexesmustusepostures
that canbe interpretedasanti-aggressive
andasapproacheliciting.
The male Peregrineexhibitshead-lowposturingmuch more frequentlythan the female,
often respondingto female movementsat a considerabledistance.Femaleintention movementsto approachthe male elicit posturescloseto horizontal,andapproachby the female
frequently causesdisplacementof the male to another perch. Althoughthe female also
exhibits these postures,vertical positionspredominate.Female displacementon male approachis not frequent.
In well-adjustedpairs,the very aggressive
Upright and Horizontal Threat posturesare
rarelyobserved
asinteractions
betweenthd mates.With newpairsthesedisplays
mayoccur
early in the season,but areobserved
lessfrequentlyasthe pair-bonddevelops.
Althoughthe
femalemay approachthe scrapein an all-horizontalpostureduringa MutualLedgeDisplay,
she immediatelyassumes
a headqowpostureif the male looks up. It is unlikely that the
horizontalposturein this caseis reallyaggressive.
Entirelyhorizontalapproachto the scrape
is usualin both sexesprior to IndividualLedgeDisplays-probablya relaxedposturefor
walkingon a ledgeor the ground.It is significantthat the female'sposturechanges
whenthe
malelooksat her;this is not seenin Gyrfalconsduringsomeinteractions(seebelow).
Posturesand vocalizationsthat are apparentlyagg.ressive
occurregularlyin the copulation
sequence
of Peregrines.
The femalepostureat this time is totally nonaggressive,
asis her
Whine vocalization.The male usesthe Chitter vocalizatiortand postureswith aggressive
components.There is little doubt of the aggressiveness
of the Chitter, at least in some
contexts.It hasbeenobservedduringUpright Threatencounters,
duringforced(by female)
Food-Transferring,
and by both sexeswhen trying to force the mate off the eggs.The
combinationof this vocalizationwith the partly aggressive
posturesjust beforemountingis
difficult to interpret.The femaleis not alwaysintimidatedby thisbehavior,althoughsheis
in a compromised
positionas the malemounts.Femalesdo occasionally
refusemountingor
aggressively
displacethe male after mounting,evenwhen CopulationSolicitationoccurred
just prior to the mountingattempt. It is alsounclearwhy the malesometimes
continuesthe
Chitter vocalizationthroughoutcopulation.
WilloughbyandCade(1964) describefor the AmericanKestrela Chitter vocalizationthat
issimilar
to thePeregrine
Chitter
in terms
of itscontextual
use,butwhichapparently
signals
"friendly
approach."
Theapparent
difference
in meaning
mayberelated
to theveryslight
sizedimorphism
in Kestrels.
Because
of similar
size,themalemaybemoreintimidating
ashe
preparesto mount, and a nonaggressive
signalmight easethe interaction.Kestrelsalsousea
ChittervocalizationduringFood-Transferring.
The copulationsequence
in Peregrines
is characterized
by a seriesof "testing"actionsand
responses.
Whenthe sequence
isinitiated,by maleor female,the femaleassumes
the Copulation Solicitationposture,often facingawayfrom the male. An alternatingseriesthen proceeds,with the femalelookingup towardthe male,the maleresponding
with the HitchedWing Display and componentsof the AgonisticHead-Low Bow, and the female then respondingwith renewedhead-lowpostures.The seriesmay be repeatedseveraltimes.In a
completedsequencethe female will maintain the solicitingpostureas the male flies to
mount. Femaleterminationof the sequenceinvolvesa shift from the nonaggressive
soliciting
postureto an anti-aggressive
Display, the Extreme Head-Low Bow. Althoughthis usually
caused
the maleto aborthismountingattempt,in two experienced
pairsthe malefrequently
mountedanyway,and copulationwasusuallycompleted.
Observations
on the incubatingbehaviorof wild Peregrines,
especially
by Nelson(1970),
permit an interestingcomparisonwith captivepairs, suggesting
a differencein the pair
24
RAPTOR RESEARCH
Vol. 11, No. 1/2
relationshipthat may be important. In nature, the female controlsthe scheduleof
incubationduty. If the maleis incubatingasthe femalearriveson the nest-ledge,
he getsup
almq.s•t_i•m•ediately
andleaves
(Nelson1970,•Wrege
unpubl.).Thereverse
situation
doesnot
necessarily
elicitfemalewithdrawal.
Exchanges
for incubation
proceed
verydifferently
in
captivePeregrines.
Althoughthe female'sdominanceis usuallyevident,eithersexmay
approachthe incubatingmate andtry to urgethe mateoff theeggs.The success
of suchan
attempt is variable.Interactionsof this form indicatea fairly dose adjustmentof the pair to
one another.Althougha dominancerelationshipdevelops,successful
breedingrequiresthat
it be stable enough for overt female aggressionto be minimal, so that the male is not
constantlyintimidatedby the female.In the wild, wheremaleavoidance
may be a frequent
responseto femalepressuring,
interactionscan be more agonistic.Interactionsfor the most
part are very shortin wild pairs.
Gyrfalconsare slightlylessdimorphicthan Peregrines.Differencesare not greatin the
relativefrequencies
of head4owpostures.Theseposturesare rare in the femaleand canbe
interpretedas a reduction in the useof head-lowposturesas approach-eliciting
signals.As
mentionedpreviouslyfor Kestrels,the male Gyrfalconis intimidatingto the female,probat•ty more so than in Peregrines.As a result,definitely antiaggressive
posturesare more usual
in some ritualized displays,as are aggressive
componentsin others. For example,Mutual
LedgeDisplaysare characterized
by constanthead-lowposturesby both birds.In contrastto
male Peregrines,
the male Gyrfalconalsomaintainsthe head-lowposturewhile solicitinga
Food-Transfer.We interpret this postureas approach-eliciting
for two reasons:the male
rarely takes food to the female,at leastearly in the season;and the femaleapproaches
in a
partly aggressive,
horizontalposture,indicatinga fear component.MalePeregrines
havevery
rarelybeenobservedin approach-eliciting
postures.
Aggressive
componentsare obviousin the femaleGyrfalcon'sprimaryCopulationSolicitation. Whenthe male is at somedistance,the femalemay initially solicitcopulationwith a
horizontalhead-lowpostureand a nonaggressive
Whine.As the maleshowsintentionmovements to mount, the femaleassumes
an entirely horizontalpostureorientedtoward the male
and givesa vocalizationwith many similaritiesto the Chitter usedin aggressive
contexts
(figs.3cand3e). Ourmalewasintimidated
by thisDisplay,alternatin•g
between
theCur_v_e•:
Neck Displayandanti-aggressive
Head-LowBow.Mountingproceeded
only whenthe female
was not oriented toward the male. Thesebehaviorpatternsare consistentwith a situation
involvingan intimidatingmale and precedingan action during which the female is in a
compromised
position.As with Peregrines,
the male precopulatory
posturesinvolveaggressive components.Althoughthe Curve-NeckDisplaystresses
a loweredbeak, the body posture is verticaland tall, accompanied
by a vocalizationapparentlyidenticalto the Chitterof
aggressive
situations.It is not clearwhy Gyrfalconsand Kestrelsdiffer in the aggressiveness
of male precopulatorybehavior.The differencemay well be relatedto the aggressive
com-
ponentsin femaleGyrfalconsolicitation.
Unfortunately,observations
of Prairie Falconsare limited; comparisonwith Peregrines
and Gyrfalconsis more difficult. In our captivepairsthe pair-bondappearedstrained,and it
is well known amongfalconersthat Prairie Falconsare the most aggressive
of the North
Americanspeciesof Falco.
There is no questionof the dominantpositionof the femalein our captives.Frequent
displacement
of the male by the femaleis characteristic
throughoutthe breedingseason.
During the nonbreedingseasonthe matesavoidone another.
The extremelypugnacious
temperament
of PrairieFalconsis not easilyexplainedin terms
of the degreeof size dimorphism,which is closeto that of Gyrfalcons.However,acom-
Spring/Summer
1977 Wrege& Cade- Behaviorof Falcons
25
parisonof the relativefrequenciesof somebehaviorsis instructivein relationto the decided
female dominance.It is possiblethat the pairs observedin this study were lesswell integratedthan sometimes
occurs.Fyfe (1972) observeda captivepair that was "at ease"with
one another.
DuringMale LedgeDisplays,evenmorethan in the other species,
the malePrairieFalcon
is awareof the female'slocationin the room. Immediatelyupon her approachfor a Mutual
Ledge Display, an extremehead-lowpostureis adopted.During the Displaythe male constantlybowsor presses
himselfalmostflat on the ledge.
FemaleSolicitationfor Copulationin Prairie Falconsinvolvesan extremehead-lowpos-
ture almostalwaysorientedawayfrom the male.Often solicitationis silent.Hitched-Wing
approachby the male is usual,but aggressive
postures
just beforemountingarenot conspicuous.
Relative frequenciesof aggressive,
anti-aggressive,
and approach-eliciting
behavior exhibited by male and female falconssupport an interpretation of female dominance.The
degreeto whichthisdominancecanbe initiatedandmaintainedby "passive"intimidationas
comparedto more overt behavioralactionsappearsto dependon the differencein size
betweenthe mates.Intimidationof the mate owingto sizedifferenceis probablythe major
factor controllingrelativefrequencies
of agonisticbehavior.The ratio of thesefrequencies
in
a givenpairdepends
on theirspecific
degree
of dimorphism
andon theirhistory,whilethe
agonistic
components
of ritualized
display
behavior
maybe relatedto thesizedimorphism
characteristic
of the species
asa whole.Preliminary
analysis
of quantitative
datatendsto
support
thesequalitative
interpretations
(Wrege
unpubl.).
4. TheFunction
of Vocalizations
inSocial
Communication.
Preliminary
analysis
of (he
vocalizations
in thesefour species
indicatesthat variabilityin the contextualuseof vocalizations can be considerable.
Intergradation
from one vocalizationto anotheris particularly
strikingin Gyrfalconsand LannerFalcons.Many courtshipvocalizations
usedby captive
Gyrfalcons
arebasedon a singlesoundunit, differingonlyin the speedwith whichthe units
arerepeated(figs.3a and3c). The vocalizations
of LannerFalcons
havenot beenanalyzed;
however,they soundvery similarto Gyrfalcons.In Peregrines
andPrairieFalconsthe structures of some vocalizationsare more complex. Variability of the vocalizationsin these
species
is in the degreeof completeness
of the basicunit and isassociated
with the motivational levelor intensityof the behavior,not with specificcontexts.
Figure 2a showsthe most commonvocalizationin Peregrinecourtship.In its complete
form the Eechiphasthree parts,but frequentlyone or more partsare deletedor repeated.
The contextsin which this vocalizationis elicitedrangefrom low intensityIndividualLedge
Displaysthroughagonisticencounters.
With the exceptionof the territorialCackvocalization and the femaleCopulationWail, all vocalizationsare usedin numerouscontexts.
Apparentlythe primary functionof vocalizations
is to communicate
the intensityof
motivation.As intensityincreases,
either the speedof unit repetitionincreases
(Gyrfalcons
and Lanners,especially),or the soundunit is fragmented,with somepartsrepeatedbeforea
newunit is initiated(Peregrines
andPrairieFalcons).
Summary
The courtshipbehaviorof PeregrineFalcons,Gyrfalcons,Prairie Falcons,and Lanner
Falconswas studiedfor the purposeof describingtheir reproductivebehavior,and, using
comparativeanalysis,to characterizethe pair relationship.Twenty pairs of falconswere
studiedfor two to four yearswith an emphasis
on PeregrineFalcons.
26
RAPTORRESEARCH
Vol. 11, No. 1/2
We found the behavioralrepertoire very similarin the speciesstudied,with at least75
percentof the posturesand displayscommonto all. Most interspecificdifferenceswere in
the frequencies
of certainDisplaypostures,andin the specificcharacteristics
of vocalization.
The function of thesebehaviorpatternsin pair integrationis apparentlythe samefor all
species,making thesesimilaritiesimportant managementtools for the captivebreedingof
falcons.Experimentalwork on speciesmost availableor amenableto manipulationcan be
usedto predict, with someconfidence,the outcomeof similarmanipulationson captive
Peregrinesor other endangeredspecies.
The vocalizations
of thesespecies
arealsosimilarin their basicstructureandfunction.All
the vocalizationsshowhigh levelsof frequencymodulation,causingthe "noisy"appearance
of the audiospectrographs
(figs.2, 3, and4). Thisbasicstructuremay be relatedto the open
habitatsutilized by thesespecies(Morton 1975). A more importantsimilarityis the intergradationof some vocalizationsand their nonspecificcontextual use. In captivity, each
species
usesmanyvocalizations
to communicate
the intensityof motivation,andveryfew to
communicatethe motivation to perform a specificaction. CopulationSolicitationand territorial defensecallsfall into the latter group.
The seasonalontogenyof reproductivebehaviorin Peregrinesfollows a predictablepattern, at leastin experiencedpairs.In pairsremainingtogetherall year, courtshipis initiated
earliereachyear for aboutthreeyears.Copulationbeginsseveralweeksbeforeegg4ayingand
probably helps to strengthenthe pair-bond.The third and fourth eggsin a dutch can be
fertilized by copulationsjust before and after the secondeggis laid. This suggests
that
artificial inseminationcan be achievedwith minimaldisturbanceto the pair, providingthe
techniqueensuresthe placementof semendirectlyinto the oviduct.
The frequencywith which displaysand posturesare expressed
is the major interspecific
differencein the behaviorof the captivefalconsstudied.Sucha differenceis consistent
with
the hypothesisthat female dominanceis a characteristicof the pair relationshipin large
falconsand is possiblynecessary
for successful
reproduction.Apparentlythe primaryfactor
that influencesthe relativefrequencyof aggressive
and nonaggressive
behavioris the degree
of sizedimorphismbetweenthe sexes.
The pugnacious
natureof falconsand the potentiallyvery seriousinjury from aggressive
encountersmay be the causesof a behavioralrepertoirewith the capacityto transmitfinely
tuned information about motivation and its intensity. The amount of information communi-
cated throughposturesis probablyvery high. Frequentlybirdschangetheir response
to their
mate when little or no changein posturewas observed.The vocalizations
in eachspecies
seemwell suitedto signalingthe intensityof motivation.
Acknowledgements
We would like to thank lames D. Weaver,Kathy Johnson,and Bill Heck for their assistance with the collectionof data, and Mike Hopiak for help in the preparationof figures.
This study was supportedin part by grant numberGB 31547 from the NationalScience
Foundationand by contributionsto The PeregrineFund. Supportwasalsoreceivedfrom the
National Rifle Association,National Wildlife Federation,Wildlife ManagementInstitute, and
the American
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